Laser-stirred powder bed fusion

ABSTRACT

An additive manufacturing process that includes providing a first layer of powdered material having a predetermined thickness; using a laser beam that follows a predetermined path to fuse a portion of the material in the first layer, wherein the predetermined path of the laser beam is a repeating circular or elliptical path which incrementally proceeds in a linear direction; providing a second layer of powdered material having a predetermined thickness; using a laser beam that follows a predetermined path to fuse a portion of the material in the second layer, wherein the predetermined path of the laser beam is a repeating circular or elliptical path which incrementally proceeds in a linear direction; repeating the previous steps until a complete part or component is created; and removing any unfused powdered material from the completed part or component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/340,719 filed on May 24, 2016 and entitled“Laser-Stirred Power Bed Fusion”, the disclosure of which is herebyincorporated by reference herein in its entirety and made part of thepresent U.S. utility patent application for all purposes.

BACKGROUND OF THE INVENTION

The described invention relates in general to additive manufacturingprocesses, and more specifically to a powder bed fusion process thatincludes laser stirring as an aspect thereof.

In general, the powder bed fusion (PBF) process utilizes either a laseror an electron beam to melt and fuse material powder for the purpose ofcreating a part or component. PBF processes typically involve spreadingmaterial in the form of powder over previously deposited layers ofmaterial. There are different mechanisms for accomplishing thisincluding, for example, the use of a roller or a blade. A hopper or areservoir positioned below or next to a powder bed is used to providefresh material powder. Electron beam powder bed fusion (EB-PBF), alsoknown as electron beam melting (EBM), requires a vacuum, but can be usedwith metals and alloys for the creation of functional parts. Laserpowder bed fusion (L-PBF), also known as selective laser melting (SLM),or direct metal laser melting (DMLM), is equivalent to selective lasersintering (SLS), but involves the use of metals rather than plastics.Also, in L-PBF, the material fully melts, whereas in SLS, the materialpartially sinters. Selective heat sintering (SHS) differs from otherprocesses through the use of a heated thermal print head for fusingmaterial powder. As before, layers are added with a roller in betweenfusion of layers and a platform lowers the part accordingly.

Laser powder bed fusion (L-PBF) is an additive manufacturing process inwhich a three-dimensional component or part is built using alayer-by-layer approach. With reference to FIG. 1, L-PBF system andapparatus 10 includes housing 12, which includes region 14 forcontaining new powder stock that is moved by powder roller 16 intopowder bed 18, which sits on build platform 20, upon which component orpart 22 is created by laser 24. L-PBF typically involves the followinggeneral steps: (i) a layer of powdered material (e.g., metal), typicallyabout 0.1 mm thick, is spread over build platform 20; (ii) laser 24fuses the first layer or first cross section of part 22; (iii) a newlayer of powder is spread across the previous layer using roller 16 or asimilar device; (iv) further layers or cross sections are fused andadded; and (v) the process is repeated until the entire part 22 iscreated. Loose, unfused powdered material remains in position, but isremoved during post processing.

Conventional laser powder bed fusion (L-PBF) utilizes a back and forthlinear hatch pattern to melt a layer of metal powder. Conceptually, theindividual hatches are straight welds that are laid side-by-side andthen stacked in subsequent layers to create a three-dimensional fullydense build. As such, materials which cannot be welded autogenouslycannot be utilized in the L-PBF process without major alterations. Forexample, high-strength aluminum alloys are plagued by cracking andporosity, which lead to unacceptable bulk materials and builds. This isthe case in multiple additive technologies including electron beamprocesses. Thus, there is an ongoing need for an L-PBF process that canbe used with materials that are normally not compatible with L-PBFsystems and methodologies.

SUMMARY OF THE INVENTION

The following provides a summary of certain exemplary embodiments of thepresent invention. This summary is not an extensive overview and is notintended to identify key or critical aspects or elements of the presentinvention or to delineate its scope.

In accordance with one aspect of the present invention, a first additivemanufacturing process is provided. This additive manufacturing processincludes providing a first layer of powdered material, wherein the firstlayer of powdered material has a predetermined thickness; using a laserthat follows a predetermined path to fuse a portion of the material inthe first layer, wherein the predetermined path of the laser creates aseries of stirred hatches in the fused material; providing a secondlayer of powdered material, wherein the second layer of powderedmaterial has a predetermined thickness; using a laser that follows apredetermined path to fuse a portion of the material in the secondlayer, wherein the predetermined path of the laser creates a series ofstirred hatches in the fused material; repeating the previous stepsuntil a complete part or component is built; and removing any unfusedpowdered material from the completed build.

In accordance with another aspect of the present invention, a secondadditive manufacturing process is provided. This additive manufacturingprocess includes providing a first layer of powdered material, whereinthe first layer of powdered material has a predetermined thickness;using a laser beam that follows a predetermined path to fuse a portionof the material in the first layer, wherein the predetermined path ofthe laser beam is a repeating oscillating path which incrementallyproceeds in a linear direction; providing a second layer of powderedmaterial, wherein the second layer of powdered material has apredetermined thickness; using a laser beam that follows a predeterminedpath to fuse a portion of the material in the second layer, wherein thepredetermined path of the laser beam is a repeating oscillating pathwhich incrementally proceeds in a linear direction; repeating theprevious steps until a complete part or component is built; and removingany unfused powdered material from the completed build.

In yet another aspect of this invention, a third additive manufacturingprocess is provided. This additive manufacturing process includesproviding a first layer of powdered material, wherein the first layer ofpowdered material has a predetermined thickness; using a laser beam thatfollows a predetermined path to fuse a portion of the material in thefirst layer, wherein the predetermined path of the laser beam is arepeating circular or elliptical path which incrementally proceeds in alinear direction; providing a second layer of powdered material, whereinthe second layer of powdered material has a predetermined thickness;using a laser beam that follows a predetermined path to fuse a portionof the material in the second layer, wherein the predetermined path ofthe laser beam is a repeating circular or elliptical path whichincrementally proceeds in a linear direction; repeating the previoussteps until a complete part or component is built; and removing anyunfused powdered material from the completed build.

Additional features and aspects of the present invention will becomeapparent to those of ordinary skill in the art upon reading andunderstanding the following detailed description of the exemplaryembodiments. As will be appreciated by the skilled artisan, furtherembodiments of the invention are possible without departing from thescope and spirit of the invention. Accordingly, the drawings/figures andassociated descriptions are to be regarded as illustrative and notrestrictive in nature.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, schematically or photographically illustrate oneor more exemplary embodiments of the invention and, together with thegeneral description given above and detailed description given below,serve to explain the principles of the invention, and wherein:

FIG. 1 is a generalized drawing of an exemplary laser powder bed fusionprocess and a laser bed fusion system and apparatus;

FIG. 2 provides laser stirring path schematics for circular andelliptical oscillation, in accordance with an exemplary embodiment ofthe present invention;

FIG. 3 depicts four aluminum substrates with 1080 individual stirringexperiments completed and powder layers removed;

FIG. 4A includes multiple cross-sectional views of stirred weldexperiments for AlSi10Mg; and FIG. 4B includes multiple cross-sectionalviews of stirred weld experiments for Al6061;

FIG. 5A includes multiple cross-sectional views of stirred weldexperiments for Al7075 (left); and FIG. 5B includes multiplecross-sectional views of stirred weld experiments for Al2024 (right);

FIG. 6 is a cross-sectional view of cracking that is typical in anAl7075 weld;

FIG. 7A is an unetched cross-sectional view a of a multilayer Al7075build with linear hatching; FIG. 7B is an unetched cross-sectional viewa of a multilayer Al7075 build with stirred hatching; and FIG. 7C is agraphic depicting the build direction of FIGS. 7a and 7B in the XZplane;

FIG. 8 is an unetched cross-sectional view of a multilayer Al6061 buildwith stirred hatching, wherein the build direction is the same as thatshown in FIG. 7C;

FIG. 9A is an unetched XZ plane cross-sectional view of a fully dense,multilayer A205 build with stirred hatching; FIG. 9B is an unetched XZplane cross-sectional view of a fully dense, multilayer aluminumscandium build with stirred hatching, wherein the build directions ofFIG. 9A and FIG. 9B are the same as that shown in FIG. 7C;

FIG. 10A is an XZ plane cross-sectional view of a multilayer Inconel 718build deposited using linear hatching; and FIG. 10B are XZ planecross-sectional views of multilayer Inconel 718 builds deposited withvaried parameter combinations using stirred hatching to alter buildmicrostructure, wherein the build directions of FIG. 10A and FIG. 10Bare the same as that shown in FIG. 7C;

FIG. 11A is an XZ plane cross-sectional view of a multilayer 316Lstainless steel build deposited using linear hatching; and FIG. 11B areXZ plane cross-sectional views of a multilayer 316L stainless steelbuilds deposited with varied parameter combinations using stirredhatching to coarsen build microstructure, wherein the build directionsof FIG. 11A and FIG. 11B are the same as that shown in FIG. 7C;

FIG. 12A is an XZ plane cross-sectional view of a multilayer 316Lstainless steel build deposited using linear hatching; and FIG. 12B areXZ plane cross-sectional views of a multilayer 316L stainless steelbuilds deposited with varied parameter combinations using stirredhatching to alter build microstructure, wherein the build directions ofFIG. 12A and FIG. 12B are the same as that shown in FIG. 7C; and

FIG. 13A is an XZ plane cross-sectional view of a multilayer Ti-6Al-4Vbuild deposited using linear hatching; and FIG. 13B are XZ planecross-sectional views of a multilayer Ti-6Al-4V builds deposited withvaried parameter combinations using stirred hatching to alter buildmicrostructure, wherein the build directions of FIG. 13A and FIG. 13Bare the same as that shown in FIG. 7C.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the present invention are now described withreference to the Figures. Although the following detailed descriptioncontains many specifics for purposes of illustration, a person ofordinary skill in the art will appreciate that many variations andalterations to the following details are within the scope of theinvention. Accordingly, the following embodiments of the invention areset forth without any loss of generality to, and without imposinglimitations upon, the claimed invention.

The present invention includes the application of laser stirring to theL-PBF additive manufacturing process (LS-PBF) and addresses inherentdifficulties in the process by using a variety of materials intraditional L-PBF including high-strength aluminum alloys. As previouslystated, such alloys often suffer from cracking and porosity, which leadsto unacceptable bulk materials and builds. The additive manufacturingindustry has researched chemistry alterations to more common alloys forspecific use in L-PBF and has searched for existing materials withequivalent or comparable material properties. This invention differsfrom these approaches in that it alters the laser weld path used in theentire L-PBF process and allows for the use of materials that have beenheavily certified and relied on in other technologies in L-PBFprocesses.

Conventional laser powder bed fusion (L-PBF) utilizes a back and forthlinear hatch pattern to melt a layer of metal powder. Conceptually, theindividual hatches are straight welds which are laid side by side andthen stacked in subsequent layers to create a three-dimensional fullydense build. As such, materials which cannot be welded autogenouslycannot be utilized in the L-PBF process without major alterations. Thepresent invention includes the application of laser beam stirring toeach of the hatches in each layer. With reference to FIG. 2 and Tables 1and 2, below, an oscillatory path was programmed into a test bed forcircular and elliptical stirring parameters. FIG. 2 provides laserstirring path schematics for circular and elliptical oscillation. Table1 provides circular stirring dimensional parameters and Table 2 provideselliptical stirring dimensional parameters.

TABLE 1 Circular Stirring Dimensional Parameters Experiment Numberwithin the Parameter Set 6 7 8 9 10 Stirring Type Circular CircularCircular Circular Circular D1 (μm) 90 180 180 270 360 D2 (μm) 45 135 90180 270

TABLE 2 Elliptical Stirring Dimensional Parameters Experiment Numberwithin the Parameter Set 11 12 13 14 15 Stirring Type EllipticalElliptical Elliptical Elliptical Elliptical D1 (μm) 90 90 180 180 180 L1(μm) 45 90 90 180 180 L2 (μm) 30 45 45 135 90

Laser stirring paths were run using a range of laser powers and travelspeeds to determine optimal parameters for multiple aluminum alloys.Results indicated that the application of laser stirring results infully dense, crack-free consolidation of AlSi10Mg, Al6061, Al7075, andAl2024 metal powder on a metal substrate. The balancing of heat input,oscillation travel speed, linear travel speed, and thermal response timeof the material being processed ensures that weld cracking and porosityare eliminated or otherwise rendered insignificant. The welded areabegins initial solidification, but does not fully solidify before theoscillation of the laser path returns to break up the dendriticmicrostructure which is in the process of forming. If tuned correctly,this invention refines the final weld microstructure while eliminatingproblems related to solidification cracking, which is due to acombination of known factors.

The present invention was tested using four different aluminum alloys.Single powder layers (about 40 micron thick) of AlSi10Mg, Al6061,Al7075, and Al2024 were placed on corresponding aluminum substrates.Each substrate and powder layer then had 270 stirring experiments andpath variations performed for a total of 1080 individual experiments,shown in FIG. 3. Of the 1080, 120 were selected for sectioning andinspection with heat inputs ranging from 0.1169 to 0.3795 W/(mm/s).FIGS. 4A-4B and 5A-5B show some of the positive results for oscillatedlaser welds with each alloy. A typical cross-section of a prior art weldwould show a high degree of cracking or porosity using these alloys, asshown in FIG. 6 for Al7075. The results presented herein (FIGS. 4A-4Band 5A-5B) show fully dense, crack free weld cross sections for threevery different laser speeds and powers in each alloy. This result isillustrative with regard to the efficacy of stirring in reducingcracking and porosity over a wide range of parameters using selectstirring paths and frequencies. The building of fully dense prisms andfurther optimization of stirring parameters are aspects of thisinvention.

The present invention was further demonstrated by building up multiple40 micron thick layers of stirred hatches which were side by side,resulting in a three-dimensional deposit. FIGS. 7A and 7B show a directcomparison between a multilayer deposit built with linear hatches and amultilayer deposit built with stirred hatches. One of ordinary skill inthe art will appreciate the reduction in cracking severity, resulting ina higher overall quality build. FIGS. 8A-8B and 9A-9B illustrate thesuccessful application of stirred hatches in multilayer deposits for Al6061, A205, and Aluminum Scandium alloys. In each case, low amounts ofcracking and porosity are observed Three A205 tensile specimens eachwere built using stirred and linear hatches to evaluate differences inmechanical properties based on the application of this invention.Stirred specimens showed an average UTS and YS of 321 MPa and 192 MParespectively in the as-built condition. Linear hatched specimensdisplayed an average UTS and YS of 295 MPa and 172 MPa respectively inthe as-built condition. For this alloy, stirred hatching resulted in anincrease in UTS and YS of ˜9% and ˜11% respectively over linearhatching. This result further illustrates the beneficial effect of thepresent invention on as-built microstructure.

The present invention was also demonstrated on non-aluminum alloysincluding Inconel 718, 316L Stainless Steel, and Ti-6Al-4V. Builds werecompleted using laser stirred hatching with up to 30 differentcombinations of laser travel speed, laser power, hatch spacing, andstirring paths. FIGS. 10A-10B show the typical as-built microstructurefor linear hatching (FIG. 10A) and three variations in microstructure(FIG. 10B) possible with stirred hatching for Inconel 718. Grainrefinement and the formation of necklace microstructure, containing bothlarge and small grains, are apparent in the stirred hatching builds.FIGS. 11A-11B and 12A-12B show the typical as-built microstructure forlinear hatching (FIGS. 11A and 12A) and four variations inmicrostructure possible with stirred hatching (FIGS. 11B and 12B) for316L stainless steel. The stirred hatching builds display larger grains,columnar grains, less elongated grains, and/or combinations of large andsmall grains depending on the stirring parameters employed. FIGS.13A-13B show the typical as-built microstructure for linear hatching(FIG. 13A) and three variations in microstructure (FIG. 13B) possiblewith stirred hatching for Ti-6Al-4V. As would be apparent to one ofordinary skill in the art, the columnar microstructure visible in thelinear hatched build can be coarsened or refined to various levelsthrough the use of stirred hatches. These positive results for threecommon L-PBF alloys show that laser stirred hatching can be used toeffectively tailor microstructure in three-dimensional builds. Grainrefinement, reduction of anisotropy, and texture alteration areaccomplished through stirring by balancing heat input, oscillationtravel speed, linear travel speed, and thermal response time of thematerial being processed to ensure that grains are broken up afterpartial solidification. The same factors can be tuned or otherwisemanipulated such that the stirred path mimics a larger spot size laserand the material experiences slower cooling rates, thereby resulting ingrain coarsening.

Important advantages and aspects of this invention include thefollowing: (i) application of a stirred laser path to L-PBF processes,which traditionally use straight laser paths (linear hatches); (ii) beampath oscillations (circular and elliptical) at frequencies up to andover 7500 Hz with oscillation widths down to 45 μm; and (iii) abalancing between heat input, oscillation travel speed, linear travelspeed, and thermal response time of the material being processed. Thisprocess provides for the use of a range of materials which are currentlynot plausible for use in L-PBF processes, such as high strength aluminumalloys of the 6XXX and 7XXX series. The process has been shown toimprove as-built material properties of builds by increasing grainrefinement in aluminum alloys. Additionally, LS-PBF has shown greatsuccess in modifying as-built microstructure compared to linearhatching, creating opportunities for location based microstructuraltailoring in builds without any required post processing steps as wellas general increases in additively manufactured material properties.

While the present invention has been illustrated by the description ofexemplary embodiments thereof, and while the embodiments have beendescribed in certain detail, it is not the intention to restrict or inany way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to any of the specific details, representative devices andmethods, and/or illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of the general inventive concept.

What is claimed:
 1. An additive manufacturing process, comprising: (a)providing a first layer of powdered material, wherein the first layer ofpowdered material has a predetermined thickness; (b) using a laser thatfollows a predetermined path to fuse a portion of the material in thefirst layer, wherein the predetermined path of the laser creates aseries of stirred hatches in the fused material; (c) providing a secondlayer of powdered material, wherein the second layer of powderedmaterial has a predetermined thickness; (d) using a laser that follows apredetermined path to fuse a portion of the material in the secondlayer, wherein the predetermined path of the laser creates a series ofstirred hatches in the fused material; (e) repeating steps (a)-(d) untila complete part or component is built; and (f) removing any unfusedpowdered material from the completed build.
 2. The process of claim 1,wherein the powdered material includes at least one high-strengthaluminum alloy.
 3. The process of claim 1, wherein the powdered materialincludes Inconel 718, 316L stainless steel, Ti-6Al-4V, or a combinationthereof.
 4. The process of claim 1, wherein the stirred hatches arecircular.
 5. The process of claim 1, wherein the stirred hatches areelliptical.
 6. The process of claim 1, wherein the stirred hatchingalters build microstructure from that achievable through linear hatchingfor metal alloys.
 7. An additive manufacturing process, comprising: (a)providing a first layer of powdered material, wherein the first layer ofpowdered material has a predetermined thickness; (b) using a laser beamthat follows a predetermined path to fuse a portion of the material inthe first layer, wherein the predetermined path of the laser beam is arepeating oscillating path which incrementally proceeds in a lineardirection; (c) providing a second layer of powdered material, whereinthe second layer of powdered material has a predetermined thickness; (d)using a laser beam that follows a predetermined path to fuse a portionof the material in the second layer, wherein the predetermined path ofthe laser beam is a repeating oscillating path which incrementallyproceeds in a linear direction; (e) repeating steps (a)-(d) until acomplete part or component is built; and (f) removing any unfusedpowdered material from the completed build.
 8. The process of claim 7,further comprising creating a predetermined balance between heat input,oscillation travel speed, linear travel speed, and thermal response timeof the material being processed.
 9. The process of claim 7, wherein thepowdered material includes at least one high-strength aluminum alloyhaving predetermined grain characteristics.
 10. The process of claim 9,wherein the process increases grain refinement in the at least onehigh-strength aluminum alloy.
 11. The process of claim 7, wherein thepowdered material includes Inconel 718, 316L stainless steel, Ti-6Al-4V,or a combination thereof.
 12. The process of claim 7, wherein therepeating oscillating path is circular.
 13. The process of claim 7,wherein the repeating oscillating path is elliptical.
 14. The process ofclaim 7, wherein the frequency of the oscillating beam path is at least7500 Hz and wherein the width of the oscillating beam path is 45 μm orless.
 15. An additive manufacturing process, comprising: (a) providing afirst layer of powdered material, wherein the first layer of powderedmaterial has a predetermined thickness; (b) using a laser beam thatfollows a predetermined path to fuse a portion of the material in thefirst layer, wherein the predetermined path of the laser beam is arepeating circular or elliptical path which incrementally proceeds in alinear direction; (c) providing a second layer of powdered material,wherein the second layer of powdered material has a predeterminedthickness; (d) using a laser beam that follows a predetermined path tofuse a portion of the material in the second layer, wherein thepredetermined path of the laser beam is a repeating circular orelliptical path which incrementally proceeds in a linear direction; (e)repeating steps (a)-(d) until a complete part or component is built; and(f) removing any unfused powdered material from the completed build. 16.The process of claim 15, further comprising creating a predeterminedbalance between heat input, laser beam travel speed, linear travelspeed, and thermal response time of the material being processed. 17.The process of claim 15, wherein the powdered material includes at leastone high-strength aluminum alloy having predetermined graincharacteristics.
 18. The process of claim 17, wherein the processincreases grain refinement in the at least one high-strength aluminumalloy.
 19. The process of claim 17, wherein the powdered materialincludes Inconel 718, 316L stainless steel, Ti-6Al-4V, or a combinationthereof.
 20. The process of claim 17, wherein the frequency of thecircular or elliptical beam path is at least 7500 Hz and wherein thewidth of the circular or elliptical beam path is 45 μm or less.